Summarizing "acceptable Wikipedia content" can be dangerous: for every rule, there may be an exception, not to mention many examples where the rule is not followed. With that said, let's summarize Wikipedia's general rules for acceptable content!
There are three "core content policies" to remember. The first is to keep a "Neutral point of view"; Wikipedia articles are supposed to be non-partisan. Any views expressed must have a source to back it up, according to "Verifiability". And just because you know something, that isn't quite enough—"No original research" means you'll need a printed source to back up your claims.
A related fourth rule: that printed source had better be solid, so make sure you're "Identifying reliable sources". Finally, don't forget that Wikipedia uses a non-standard form of copyright, so anything you write becomes free for others to use later.